Dewaxing is one of the more important processes used in the refining of hydrocarbon oils, since removal of the wax results in an oil of markedly improved pour point. The process is usually carried out by chilling the oil to a sufficiently low temperature in order to precipitate the wax, and then filtering the wax from the oil. It is common practice to add to the oil solvents which tend to dissolve the oil and precipitate the wax. After the waxy constituents of the oils have precipitated, there is a marked tendency for the wax crystals to block the filters during the subsequent filtration step. This blockage considerably increases the time of filtration and also the amount of oil trapped in the wax cake.
United Kingdom patent No. 1,145,427, complete specification published Mar. 12, 1969, and whose disclosure is incorporated herein by reference, discloses that the above-described dewaxing process can be considerably improved by precipitating the wax in the presence of a polyalkyl acrylate of which the average number of carbon atoms in the alkyl side chains is at least 14. The presence of only small amounts of these polyalkyl acrylates is sufficient to improve the filtration rate.
Further work indicates that at least one of the polyalkyl acrylates appears to need a solvent to oil ratio of three or greater at the point of incipient crystallization. Where a single dilution of charge with solvent is employed, this ratio may be reached. However, where multiple dilutions are employed, i.e., multiple stages with incremental quantities of solvent added during chilling in each stage, even though the total solvent employed is supplied in a ratio of three or greater, it is unlikely that the ratio of solvent to oil at the onset of crystallization reaches three. For example, in one process employing six dilution stages, the highest dilution ratio employed in a given dilution stage is 0.9. A need has existed, therefore, for some modification of multiple dilution procedures which would allow the use of the polyalkyl acrylate modifiers and provide the benefits accruing from their use. The invention satifies that need.